This invention relates generally to apparatus for and a method of metering the application of coating material to a moving web, and more particularly, to a device for and a method of controlling and assuring that a uniform thickness of a coating composition is applied on the coated side of a moving web of material, such as linerboard, paperboard, paper, film, foil or the like, by a rod coater.
Numerous devices exist for metering excess applied coating on a moving web into a smooth thin layer. These include doctor blades for scraping the excess coating material from the web, and rods, which attempt to uniformly contact the excess coated web across its width to strike off any excess coating from the web. Examples of rod metering devices are shown in U.S. Pat. Nos. 2,229,620 and 2,774,329, while doctor or scraper blades are illustrated in U.S. Pat. Nos. 2,729,192 and 3,113,890.
In rod metering devices, the web can simply be run horizontally across the rod in tangential relation thereto or can be wrapped around spaced portions of the rod to provide a positive force downward on the rod in an attempt to assure contact along the width of the web. The latter type of metering rod is commonly referred to as a Meier rod.
One of the problems associated with the use of a Meier rod, or for that matter any metering rod, is that the amount of coating metered by the rod and applied to the web is dependent on the tension on the ends of the web or, equivalently, the net downward force of the web on the rod. If the force is not uniform across the web, there will be an uneven coating thickness across the web width from edge to edge because the rod will meter more coating off the web at the taut spots than at the loose spots. The result is a non-uniform coatweight, which is very undesirable in subsequent processing and use of the web.
Another problem is presented when the web has a slack area in it. A slack spot is one where the paper is puckered, blistered, or raised. The slack spot can be an actual rise or sag in the web and can be of considerable size. The difficulty of running a slack spot in a web over a metering rod is that an excess of coating will be applied to the web in the slack area and the rod will not meter it off. The result is a wet streak of coating on the web, which will result in rejected product, blocked and uneven rolls, poor roll quality, and machine downtime for idler roll cleanups.
The apparatus and method of the present invention is intended to provide a net loading on the rod uniformly and directly on the web, independent of web tension, which will alleviate the aforementioned problems.
Attempts have been made heretofore in the prior art to accomplish this objective. In U.S. Pat. Nos. 3,683,851 and 3,029,779 a metering rod has its support in contact with a pneumatically inflatable, flexible tube to apply pressure along the rod to cause it to uniformily contact the excess coated web. In U.S. Pat. No. 3,029,779 the tube is in contact with and beneath an intermediate support block, while in U.S. Pat. No. 3,683,851 the tube presses against a cantilevered arm carrying the support for the metering rod. Neither of these arrangements are entirely satisfactory because the pressure tube is not in direct downward contact with the metering rod along its entire length, but only is in contact upwardly with an intervening support, and thus any irregularities in the support surface upon which the tube bears and rotates results in non-uniform pressure being exerted along the length of the rod. In the case of a cantilevered support, as in U.S. Pat. No. 3,683,851, the pressure on the rod will increase the further the rod is from the fulcrum point, again resulting in non-uniform pressure along the rod length and defeating the purpose of the device.
The only attempt to make direct contact with a coating metering device to apply a uniform force to the device is illustrated in U.S. Pat. No. 3,722,465 wherein an inflatable, pneumatic tube impinges directly upon a planar doctor blade or scraper, but not a rod metering device. When used with a rod metering device the inflatable tube should contact the rod directly but the rod must be properly supported for rotation. Accordingly, the arrangement of a doctor blade and pneumatic pressurizing tube in direct contact therewith on the coated side of the web as suggested in U.S. Pat. No. 3,722,465 could not find application in the combination of a rod metering device and pressurizing tube as the rod support would be inadequate, unless there is an intervening support member as in U.S. Pat. No. 3,029,779, to support the rod and permit its rotation.